737NGX Slowing Way Early?

[Warren Allred 0]

So in a nutshell in full LNAV/VNAV mode on scheduled descent still 4 legs away from slowing to approach speed (scheduled on FMC is 240 - 200 - 173 - 147 - 138) and it has already slowed to 138...????

IAS/MACH MCP is blank as usual.

Did something to force the aircraft to slow?  What did I do?  🙂

[threexgreen 3,133]

A bit more context would make it easier to say what's going on.

[BlueStar 323]

5 hours ago, Warren Allred said:

So in a nutshell in full LNAV/VNAV mode on scheduled descent still 4 legs away from slowing to approach speed (scheduled on FMC is 240 - 200 - 173 - 147 - 138) and it has already slowed to 138...????

IAS/MACH MCP is blank as usual.

Did something to force the aircraft to slow?  What did I do?  🙂

What is showing in the speed window when you press spd inv ?

I personally use FLCH when below 10,000.  That way the magic doesn't disappoint me. 🙂

Grace and Peace, 

[Stearmandriver 2,205]

Did you configure early? When in VNAV and you select flaps, VNAV will command the min safe speed for that flaps setting, because it assumes you want to slow earlier than what the box calculated, or you wouldn't be configuring early. 

For instance, if the box is showing 195kts for a leg but you select flaps 5, the speed bug will jump to 5, probably around 175ish.  Once you select flaps 25, the speed bug will jump to your target speed. 

[Stearmandriver 2,205]

In reality, using Lvl Change to descend via an RNAV arrival is a pretty big degradation of safety, it creates much larger opportunities to bust a restriction.  Of course that's irrelevant in the sim..  but VNAV does exist for good reasons.  It's got a learning curve and is quirky in real life as well as the sim, but once you understand it well enough to know how it will react to any changes you introduce, it's a great tool. 

[BlueStar 323]

1 hour ago, Stearmandriver said:

In reality, using Lvl Change to descend via an RNAV arrival is a pretty big degradation of safety, it creates much larger opportunities to bust a restriction. 

Do you have any facts or real world experience to support this statement? 🙂

Grace and Peace, 

Edited February 16, 2019 by Bluestar
clarification

[Stearmandriver 2,205]

Yes. Been flying at the airlines for 20 years.  

[Stearmandriver 2,205]

Weird, duplicate post. 

Edited February 16, 2019 by Stearmandriver

[BlueStar 323]

41 minutes ago, Stearmandriver said:

Yes. Been flying at the airlines for 20 years.  

One thing for sure is I know where you don't work. 🙂

Grace and Peace, 

[Stearmandriver 2,205]

Um..  ok? 

Edit: It occurs to me though, that,  setting aside the somewhat odd question of where I don't work, I could have offered an explanation.

When you're in vnav on a "descend via" procedure, you have max protection against violating an altitude or speed restriction.  If you use level change and manually step down your altitudes and speeds via the MCP, you run two risks: 1. An unfortunately timed distraction could easily result in failure to promptly set the next altitude / speed,  and 2. No protection at all for an "at or below" altitude restriction.

Aggregate ASAP data shows a meaningfully increased threat of altitude and speed deviations on these procedures (and non-precision approaches) when vnav is not used. 

Once you're "descend and maintain", or if you're just getting vectored, sure, level change is great.  And I'm not saying you *can't* fly the "via" procedures in level change... but you're giving up protections for no reason. 

Edited February 16, 2019 by Stearmandriver

[Ephedrin 418]

5 hours ago, Stearmandriver said:

Um..  ok? 

military, vietnam, against Mig21. 

[Stearmandriver 2,205]

5 hours ago, Ephedrin said:

military, vietnam, against Mig21. 

What does that have to do with vnav on a "via" procedure?  I'm a little puzzled at the direction of this thread...

[vc10man 732]

Thanks for that lovely semi-tutorial', Andrew. Despite having used the 737NGX quite extensively from Day 1, I have nevertheless learned a bit afresh.👍

[MarkJHarris 128]

He's also right.

In my UK outfit, we do almost and train the newbies to use VNAV, but where an arrival procedure has step down, the MCP ALT is wound down in steps too. That's a useful SA and safety decision. As long as you wind it down approaching the restriction when it's clear it's happily aiming for it. (I use the VSD whenever I'm in an aircraft fortunate to have it). This also helps the mental model of where you're going. Our company used to be a -300 outfit, and a lot of my more senior colleagues thing the VSD and FPV are devil's toys and refuse to use them!

 

Anyway, VNAV is very much a case of rubbish in/rubbish out, so you must be clear what is in and what it's doing. Andrew gave a very good description of the logic, and how it can go pearshape if you interfere without the knowledge to correct. Changing into LVL CHG is usually the sign you've maxed out your understanding...

 

Nothing wrong and no critisism!! By reverting, you've regained control and protected yourself from the "what's it doing now?" epitaph on the end of many CVR recordings pulled from wreckage.....

So, Some things you must always bear in mind. The 737 program inside the FMC will calculate from the CF fix at flap 5 speed back up the slope to ToD. it invariably falls short, i.e. at idle thrust, your speed will decay slightly from target.  This is a safer thing than not being able to slow down! Having an aircraft that descends (at ECON) within its initial flap extension speed limits is a good thing.

As Andrew says, early flap extension will trigger the VNAV target speed to shift to that flap maneuver speed. You can, of course, use SPD INT but beware, there truly are dragons behind that gate.....

One other final thought. You're decending in ECON, past the speed reduction segment and moving towards the UP bug. When you select FLAP 1, why does the autothrottle pick up and give you a burst of power? I've never figured this out... The speed bug drops instantly to the 1 mark, yet power comes up, only for a second or two. Not sure if the PDMG model does it, but the real ones do. Always crack a smile when it happens.

I can't wait for the NG3 when it finally comes. Till then I'm stuck without my NGX as it's an FSX version and I've given up on that program.. 

[downscc 1,327]

7 hours ago, MarkJHarris said:

You're decending in ECON, past the speed reduction segment and moving towards the UP bug. When you select FLAP 1, why does the autothrottle pick up and give you a burst of power? I've never figured this out... The speed bug drops instantly to the 1 mark, yet power comes up, only for a second or two. Not sure if the PDMG model does it, but the real ones do.

I seen this and wondered if it was a quirk in the PMDG simulation... interesting.